Sol–gel systems with controlled structure formed in surfactant media

Abstract

The structure and mechanisms of formation of silica and zirconia gels produced by alkoxide hydrolysis [tetramethylorthosilicate (TMOS) and Zr (OPrⁱ)₄)] in different surfactant media have been investigated using small-angle neutron scattering and other techniques. Inverse micelle and lamellar phase systems have been formed with non-ionic octylphenyl polyether alcohol surfactants, C₈ΦE_x, where x, is 5 and 10, respectively. In these three-phase systems (water–surfactant–organic solvent) a large proportion of water is bound to the hydrophilic polyoxyethylene chains of the surfactant molecules. This has an important role in controlling the formation of the gel and its structure. Inverse micelles act as nucleation sites for the formation of very small oxide particles (⩽3 nm), which subsequently aggregate to give a fractal structure (D≈ 2.3). The lamellar phases exist as microdomains (0.1–1 µm), and alkoxide hydrolysis occurs predominantly in the surrounding zones, to give fractal aggregates (D≈ 1.9) corresponding to diffusion-limited cluster aggregation (DLCA). Under shear, such lamellar phases are aligned and then produce oriented silica gels with an anisotropic structure. Cylindrical micelles can be formed with the cationic n-alkyl trimethyl ammonium surfactant. With these systems hydrolysis of TMOS occurs in the aqueous phase surrounding the micelles. Such relations can induce the organisation of the micelles to a hexagonal phase. Subsequent elimination of surfactant results in silica xerogels containing cylindrical pores of controlled size.